Photolithography is conventionally employed in the fabrication of integrated circuits on semiconductor substrates to pattern the semiconductor substrate or metal, semiconductor, or dielectric layers formed thereon.
Photolithography involves the formation of an optical image on a layer of photosensitive material, i.e., photoresist, which is formed over an underlying material to be patterned. The optical image corresponds to the pattern to be transferred to the underlying material. Ultra-violet (UV) light is conventionally employed to form the optical image on the layer of photoresist. The optical image comprises bright regions where UV light is incident on the layer of photoresist and dark regions where no UV light is incident on the layer of photoresist. Accordingly, some portions of the photoresist are exposed to light. Other portions of the photoresist remain unexposed.
After being exposed, the layer of photoresist is developed to form a patterned layer of photoresist having a shape which is delineated by the optical image. The patterned layer of photoresist is employed as a mask to facilitate the transfer of the pattern to the underlying material.
Conventional semiconductor fabrication processes typically employ g-line photolithography, i-line photolithography, or DUV photolithography which utilize UV light having wavelengths of 0.436, 0.365, or 0.280 micrometers, respectively. The minimum size of a feature that can be formed in the underlying material, i.e., the minimum feature size, depends on the photolithography process employed. In particular, the minimum feature size is physically limited by the wavelength of light used to create the optical image. Short wavelength light provides a higher resolution optical image than long wavelength light. Accordingly, DUV photolithography provides higher resolution than i-line photolithography or g-line photolithography.
As device geometry gets smaller (.ltoreq.0.35 micrometers), the resolution provided by conventional i-line and g-line photolithography becomes inadequate to achieve minimum feature size requirements. Thus, for patterning small geometries less than about 0.35 micrometers, DUV photolithography is required.
Conventional novelac resin-based resists are typically employed for i-line and g-line photolithography. However, conventional novelac resin-based resists cannot be employed with DUV photolithography. Rather, DUV photoresist, typically comprising polysulfone-based material, is conventionally employed. This DUV photoresist is substantially transparent to DUV light. DUV light which passes through the DUV photoresist, however, will be reflected and scattered from topographical structures formed in the underlying material. In general, the underlying material is particularly reflective to DUV light.
The enhanced reflectivities in the DUV region, however, adversely impact the patterning of the underlying material to be patterned. DUV light incident on the layer of photoresist that is reflected and scattered causes unintended exposure of the layer of photoresist. This unintended exposure of the layer of photoresist leads to reflective notching. Reflections from adjacent areas produce unwanted exposure of the DUV photoresist and trigger a chemical reaction which will make the DUV photoresist soluble in aqueous media. For small feature sizes, the unintended exposure changes the critical dimension.
In order to minimize the effects of unintended exposure of the layer of photoresist due to reflected DUV light, a layer of anti-reflective coating (ARC) is required between the layer of photoresist and the underlying material to be patterned. The layer of ARC typically consists of spin-on organic material. This spin-on organic material comprises organic long-chain or polymeric compounds. In particular, for DUV photolithography, a polyimide-like ARC material is conventionally employed as a layer of ARC. Generally, these organic polymeric compounds comprising polyimide-like material contain a substantial number of aromatic rings in the polymeric compound for increased absorption of DUV light.
The layer of ARC is typically thin in comparison with the layer of photoresist. The thickness of the layer of ARC is approximately 1000 .ANG..
The layer of ARC also acts as a barrier layer to prevent chemical poisoning of the layer of photoresist by the underlying material. The DUV photoresist comprises organic material, such as polysulfone-based material. Conventional DUV photoresists, like polysulfone-based resists, are particularly sensitive toward several materials commonly used in the semiconductor industry. In particular, metals, silicon nitrides (SiN), metal nitrides (e.g., TiN), silicides or salicides (e.g., TiS and WSi), as well as doped oxides or doped glasses (e.g., boron-phosphorus doped silicate glass) can poison the DUV photoresist unless a layer of barrier material is provided for protection. APEX resist, available from Shipley Company (Marboro, Mass.) is an example of a typical polysulfone-based DUV photoresist. APEX resist is sensitive toward materials commonly used in the semiconductor industry.
To pattern the underlying material once the layer of photoresist has been developed, the exposed ARC material has to be etched away. Removing the layer of ARC will expose the surface of the underlying material. The layer of ARC can be removed ex situ or in situ. The layer of ARC is preferably removed in situ.
Etching processes such as reactive ion etching or other ion-assisted etch processes employing high density plasma are typically used to remove the layer of ARC. A mixture comprising oxygen and halocarbons, such as freons, is conventionally employed. This mixture may also include Ar.
Using a mixture consisting of oxygen and halocarbons to etch the exposed ARC material, however, adversely affects the photoresist profile. In particular, the photoresist profile is eroded or altered, thereby changing the critical dimensions. Also, when oxygen and halocarbons are employed, the DUV photoresist, which comprises polysulfone-based material, is etched at a faster rate than the layer of ARC.
Additionally, employing a mixture comprising oxygen and halocarbons to etch the layer of ARC often leaves behind residues. Etching with halocarbons tends to result in the formation of polymers. These polymers may be deposited on the sidewalls of the photoresist or on the surface of the underlying material to be patterned. The deposition of these polymers on the sidewalls of the photoresist can cause CD variation. The residues left on the surface of the underlying material impede etching thereof. This effect, whereby residues left on the surface of the underlying material impede etching, is conventionally known as micromasking.
The addition of oxygen can reduce the formation of these polymers. High O.sub.2 concentrations, however, can produce severe erosion of the photoresist and consequently large variation in CD. Accordingly, an appropriate concentration of oxygen must be employed which provides a balance between the advantageous tendency of oxygen to reduce the formation of residues with the disadvantageous tendency of oxygen to erode the photoresist.
Alternatively, pure oxygen or oxygen mixed with Ar can be used to etch the exposed ARC material. Etching processes employing pure oxygen or oxygen mixed with Ar are residue-free. By residue-free is meant that after removal of the layer of ARC, no residues are left on the surface of the underlying material to be patterned. Employing pure oxygen or oxygen mixed with Ar to etch the ARC material, however, causes substantial erosion of the layer of photoresist.
As described above, both DUV photoresist and the layer of ARC comprise organic materials. For DUV photolithography, the layer of ARC typically comprises a polyimide-like material and the layer of photoresist typically comprises a polysulfone-based resist. Since the DUV photoresist and the layer of ARC are both organic polymers, highly selective removal of the layer of ARC by plasma etching techniques is not possible. For example, employing pure oxygen or oxygen mixed with Ar in the etching process causes organic material to be removed from both the layer of ARC and the DUV photoresist. In fact, a layer of photoresist comprising polysulfone-based photoresists etches at a higher rate than the layer of ARC comprising polyimide-like material.
Additionally, the critical dimension of the photoresist mask, i.e., the layer of patterned photoresist, is altered by lateral erosion. This lateral erosion is caused by the isotropic etching of the patterned photoresist by the oxygen. Argon bombardment, also, causes substantial faceting of the top corners of the features in layer of photoresist. This faceting ultimately can cause degradation of the photoresist profile.
An etching process which employs nitrogen, a non-reactive gas, mixed with inert gases (e.g., helium, neon, and argon, etc.) for etching ARC material, is the subject of a separately-filed patent application, Ser. No. 08/359,232, filed on Dec. 19, 1994, by Subhash Gupta et al, entitled "Selective i-line BARL Etch Process" B-056!. This etching process is specifically designed for i-line photolithography applications wherein an organic i-line photoresist material and an organic i-line bottom anti-reflection layer (BARL) material are employed to pattern an underlying layer. The total carbon and oxygen content is different in the organic i-line photoresist material and the organic i-line BARL material. In particular, the organic i-line BARL material has a higher concentration of oxygen atoms than the organic i-line photoresist material. As such, the organic i-line BARL material can be made more polarized than the organic i-line photoresist material. The polarizability affinity of organic i-line BARL coating material originates from the anhydride functional groups in the moiety. The etching process disclosed in the above-mentioned patent application which employs nitrogen mixed with inert gases to etch the organic i-line BARL material makes use of the polarizability affinity of organic i-line BARL material to selectively etch the organic i-line BARL material at a higher rate than the organic i-line photoresist material. An etching selectivity of organic i-line BARL to organic i-line photoresist of 1.6 is achieved.
Thus, there remains a need for an etching process for DUV photolithography which etches the layer of ARC with minimal erosion or alteration of the photoresist mask, and which is residue-free, thereby avoiding micromasking.